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Experimental investigation of the effect of flapping on

the lift and thrust forces of 3D-wing

Mojtaba Ramezani Voloojerdi1, Mahmoud Mani 2*

1 PHD candidate, Aerospace Engineering Department, Amirkabir University of Technology, Tehran, Iran 2Prof, Aerospace Engineering Department, Amirkabir University of Technology, Tehran, Iran

ABSTRACT

Effects of flapping on lift and thrust forces in a 3D flapping wing has been investigated at low Reynolds

numbers and several reduced frequencies, using experimental tests in a subsonic wind tunnel. Tests have been

performed at Reynolds numbers 42000 to 170000 and reduced frequencies 0 to 0.45 that most birds flight at

this ranges. Also the ranges of angle of attacks are between 0°-24°. Results has shown that increase of reduced

frequency can enhance the lift force by up to 100 percent and in some cases reduce drag force to zero.

Furthermore, increment of reduced frequency has caused delay at stall of wing. Also by increasing the

Reynolds number from 42000 to 86000, major region of the boundary layer of wing surface becomes turbulent,

so maximum lift force increases by 40 percent. Wind tunnel test results show that the effect reduced frequency

on the lift force was dependent on the angle of attack, so at the lower attack angles, the increase of reduced

frequency did not affect the lift coefficient, but, with increment in angle of attack, the positive effect of the

reduced frequency on the coefficient of the lift force increased.

KEYWORDS

Flapping wing, Low Reynolds numbers, Wing aerodynamics, Subsonic flow, Micro air vehicle.

* Corresponding Author: Email: mani@aut.ac.ir

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1. Introduction

Shortened takeoff and landing distances and lowered

stall speeds, a characteristic of birds’ flights, would be

achievable with improved aerodynamic performance

including higher lift coefficient at lower incidence

angles and delayed stallSelig and Guglielmo designed

and analyzed new high-lift S1223 airfoil [1]. This airfoil

which has CLmax = 2.2 at Re = 2×105 is one of the most

known bird-like airfoils.

In other hand flapping of wing change the aerodynamic

properties of wing by providing thrust force and

increasing lift force. Some research about flapping

flight focused on aerodynamic properties of membrane

wings [2-7]. Thrust force in membrane wings is more

than thick wings but lift force of thick wings is more

than membrane wing, so thick flapping wings can

provide large capacity of payload in flight. Therefore,

aerodynamic properties of thick airfoil in flapping flight

subject of many researches. [8-13]

According to previous studies, testing and analyzing

a thick airfoil flapping wing that is very similar to cross

section of bird’s wing are mainly considered. Bird-like

S1223 airfoil is chosen for flapping wing in order to be

similar to bird wings cross section. All experiments are

conducted in different angles of attack for finding the

aerodynamic characteristics of flapping wings along

with the analysis of thrust and lift coefficients.

2. Experimental methods and facilities

2.1 Test models

Test model consist of two parts. The first is a

straight wing that inspired by nature, the S1223 airfoil

has been selected as the cross-section of this. The aspect

ratio of wing is 4.1. The second is flapping mechanism

that includes electric motor, gearbox, shaft and rods.

The characteristic of flapping model in comparison of

typical pigeon [8] was illustrated in table 1.

2.1 Experimental setup

Experiments were conducted in a subsonic, open

loop wind tunnel in the aerodynamic research laboratory

at the Amirkabir University of Technology with

rectangular 1m ×1m cross section and having a 1.8m

length test section. For boundary layer growth along the

tunnel walls, the test section is diverged by 1 degree.

The minimum and maximum velocity of the tunnel is

2.5 m/s and 60 m/s respectively that can be achieved via

a 100KW alternating current electric motor. To ensure

good flow quality in the test section, the tunnel settling

chamber contains a 4mm thick honeycomb and three-

layer anti-turbulence screens. The maximum turbulence

intensity is less than 0.2% measured by hot-wire

anemometry.

A view of the test bed and the model can be seen in

Figure 1. The external force balance was used to detect

the aerodynamic lift and drag forces of the wings. The

load limits are 240N along the normal axis (lift) and

80N on the horizontal axis (drag). In addition, the

minimum resolution along the horizontal axis is 0.001N,

and along the normal axis, it is 0.003N. Also, the

hysteresis of lift and drag forces are less than 0.1 and

0.15 percent respectively. The model is mounted on the

angle of attack setting device and this device is placed

on the external force balance as shown in Figure 2. As

illustrated in Fig.1c the analog voltage signals of force

balance are transmitted to a personal computer by an

amplifier and a data acquisition board. The data was

collected at a sample rate of 10 KHz for 7 seconds.

Table 1. Geometric and kinematic characteristics of the

Test model and pigeon

Parameter Test

model pigeon

Wing chord

0.15 m 0.11 m

Wing span

0.8 m 0.66 m

Wing area

0.1 m 0.062 m

Aspect ratio

4.1 7.2

Flapping frequency

0-5 Hz 8 Hz

Reduce frequency

0-0.45 0.25

Flapping amplitude

30° 0° -90°

Air speed

5-20 m/s 11 m/s

Figure 1. Model in test section

External force balance

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Figure 2. Schematic of experimental setup

3. Results and Discussion

The results of the wind tunnel test are presented in

this section. Aerodynamic forces on flapping wing are

measured in 0°-24° angles of attack and Reynolds

numbers 4.3×104, 8.6×104, 1.3×105, and 1.7×105. The

results are investigated in two subsections. In the first

subsection, the effects of reduce frequency on Lift

coefficient, CL and thrust coefficient, CT, are discussed.

In the second subsection, the effects of increasing

Reynolds number on CL, CT are analyzed. The

blockage of flow by walls would not occur as the span

of the wing is less than 80 cm, and test section width of

the wind tunnel is 1m.

The Reynolds number and reduce frequency are

given by equation 1 and equation 2:

ReVC

(1)

fCK

V

(2)

Where ρ is the density of air, V is the airspeed, C is the

cord of wing, µ is the coefficient of air viscosity, and f

is the flapping wing frequency.

3.1. Effects of reduce frequency

The effect of reduce frequency on lift and thrust

coefficients shown on Figure 3 for Re=42000. Based on

Figure 3 increment of reduce frequency, increases the

lift force up to 100% and increase the stall angle, also

reduce the thrust force.

The effect of angle of attacks on lift and thrust force

coefficients shown on Figure 4. In low angle of attacks,

reduce frequency is not affected on lift force but in high

angle of attacks lift force increases by increment of

reduce frequency due to reattachment of separated flow

on the wing.

3.2. Effects of Reynolds number

The effects of Reynolds number on lift and thrust

force for k = 0.1 shown on Figure 5. As shown in this

figure the increment of Reynolds number from 42000 to

86000 increases the lift force and the stall angle due to

the enlargement of the turbulent boundary layer on the

wing.

Figure 3. Effects of reduce frequency on force coefficients

Figure 4. Effects of angle of attacks on force coefficients

Figure 5. Effects of Reynolds No. on force coefficients

4. Conclusions

A flapping bird-inspired wing platform is proposed

and investigated experimentally. The main result of this

investigation mentioned as below.

- Increment of reduce frequency increases stall angle

and lift force up to 100% for Re=42000.

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- In low angle of attack increment of reduce frequency

increases thrust force but not affected on lift force

while in high angle of attacks not affected on thrust

force and increases the lift force.

- The increment of Reynolds number from 42000 to

86000 increases the lift force the stall angle due to

the enlargement of the turbulent boundary layer on

the wing.

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